Dental adhesive

ABSTRACT

Non-aqueous dental adhesive comprising a mixture containing
     (i) one or more polymerizable monomers optionally containing an acidic group,   (ii) a polymerization initiator, and   (iii) a thermal polymerization inhibitor of the following formula (I):   

     
       
         
         
             
             
         
       
         
         
           
             wherein 
             R′ 1  represents
           a hydrogen atom, or a saturated hydrocarbon group having 1 to 18 carbon atoms.   
         
             R′ 2 , which may be the same or different if more than one R′ 2  is present, independently represent
           a saturated hydrocarbon group having 1 to 18 carbon atoms, and   
         
             c represents an integer of from 1 to 4, and 
           
         
         (iv) optionally an organic solvent.

FIELD OF THE INVENTION

The present invention relates to a dental adhesive having improvedstorage stability and low toxicity. The dental adhesive may be aone-pack dental adhesive composition, in particular a total etch dentaladhesive. The present invention also relates to the use of a specificthermal polymerization inhibitor in a dental adhesive composition.

BACKGROUND OF THE INVENTION

Dental adhesive compositions known from the prior art typically containa mixture of a polymerizable monomer and an initiator system in asuitable solvent. The activity of the polymerizable monomers and theinitiator system of the mixture must be adapted to provide sufficientcuring activity and adhesion on dentin and enamel surfaces. However, anincreased activity leads to a complex stability problem during storageof the components of the mixture. Specifically, the initiator system maybe activated leading to premature polymerization of the mixture.

As a result of the stability problem of the mixture, the storagestability at room temperature of commercial dental adhesive compositionsknown from the prior art may be insufficient. For example, conventionalcommercial one-part self-etching, self-priming dental adhesivecompositions must be stored in a refrigerator in order to avoiddeterioration by solvolysis or polymerization. The commercialcomposition “iBond Gluma inside” may be mentioned, which has a lowthermal stability when stored at temperatures of 37° C. or 50° C. due topremature polymerization within less than two weeks, which is indicativeof an insufficient thermal stability at room temperature for allpractical purposes. Similar stability problems are observed with otherdental adhesive compositions.

EP-A 1 548 021 suggests hydrolysis stable one-part self-etching,self-priming dental adhesive compositions containing specific monomershaving improved resistance against hydrolysis under acidic conditions.In order to improve the stability of the initiator system, EP-A 1 548021 suggests a stabilizer such as hydroquinone monomethylether,2,6-di-tert.-butyl-p-cresol, tetramethyl piperidine N-oxyl radical andgalvanoxyl radical. However, generic one-part self-etching, self-primingdental adhesive composition known from EP-A 1 548 021 still requireimprovement of the thermal stability at storage for attaining astability of at least 10 days at 60° C. required. Moreover, hydroquinoneis an allergenic compound imparting undesirable toxic properties to adental adhesive composition.

SUMMARY OF THE INVENTION

It is a problem of the present invention to provide a dental adhesivecomposition having a low toxicity and thermal stability at storage of atleast 10 days at 60° C.

Moreover, it is the problem of the present invention to provide aspecific class of compounds which may be used to stabilize a dentaladhesive composition for at least 20 days during storage at 60° C.

The present invention provides a non-aqueous dental adhesive comprisinga mixture containing

(i) one or more polymerizable monomers optionally containing an acidicgroup,(ii) a polymerization initiator,(iii) a thermal polymerization inhibitor of the following formula (I):

-   -   wherein    -   R′₁ represents        -   a hydrogen atom, or a saturated hydrocarbon group having 1            to 18 carbon atoms.    -   R′₂, which may be the same or different if more than one R′₂ is        present, independently represent        -   a saturated hydrocarbon group having 1 to 18 carbon atoms,            and    -   c represents an integer of from 1 to 4, and        (iv) optionally an organic solvent.

The present invention is based on the recognition that a mixturecontaining

one or more polymerizable monomers optionally containing an acidicgroup, one or more organic or inorganic acids, and a polymerizationinitiator is problematic with regard to polymerization wherebyconventional stabilizers such as hydroquinone monomethylether,2,6-di-tert.-butyl-p-cresol, tetramethyl piperidine N-oxyl radical andgalvanoxyl radical provide an insufficient effect for attaining a highstorage stability.

The present invention is furthermore based on the recognition that aspecific class of water insoluble stabilizers provides a surprisingstabilizing effect so that a dental adhesive may be provided which hasan excellent storage stability due to an improved resistance againstpremature polymerization.

Accordingly, the present invention also relates to the use of a compoundof the following formula (I):

-   -   wherein    -   R′₁ represents        -   a hydrogen atom, or a saturated hydrocarbon group having 1            to 18 carbon atoms.    -   R′₂, which may be the same or different if more than one R′₂ is        present, independently represent        -   a saturated hydrocarbon group having 1 to 18 carbon atoms,            and    -   c represents an integer of from 1 to 4,        as a thermal polymerization inhibitor in a dental composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The dental adhesive composition according to the present inventioncontains a water-insoluble thermal polymerization inhibitor of formula(I). Preferably, the saturated hydrocarbon group which may be present asR′₁ or R′₂ in formula (I) represents a straight chain or branched C₁₋₈alkyl group or a C₃₋₈ cycloalkyl group optionally substituted by one ormore C₁₋₅, alkyl groups or a C₄₋₁₈ cycloalkylalkyl group optionallysubstituted by one or more C₁₋₅ alkyl groups.

Preferably, R′₁ represents a straight chain or branched C₁₋₁₈ alkylgroup. In a preferred embodiment, R′₁ is hydrogen or a tert.-butylgroup.

R′₂ in formula (I) is believed to provide a steric effect due to thebulky nature of the substituent in this position. Therefore, at leastone R′₂ in formula (I) is a saturated hydrocarbon group having 1 to 18carbon atoms. Accordingly, in a specific embodiment, at least one R′₂ informula (I) represents a branched C₃₋₁₈ alkyl group or a C₄₋₈ cycloalkylgroup optionally substituted by one or more C₁₋₅ alkyl groups or a C₄₋₁₈cycloalkylalkyl group optionally substituted by one or more C₁₋₅ alkylgroups. More specifically, at least one R′₂ in formula (I) preferablyrepresents a branched C₃₋₁₈ alkyl group or a C₃₋₁₈ cycloalkyl groupoptionally substituted by one or more C₁₋₅ alkyl groups. Even morespecifically, at least one R′₂ in formula (I) preferably represents abranched C₃₋₈ alkyl group. In a further preferred embodiment, R′₂ is atert.-butyl group.

c represents an integer of from 1 to 4, preferably 1 or 2. In a specificembodiment, c is 1.

Preferably, thermal polymerization inhibitor is a compound of thefollowing formula (I′):

wherein

R′₁ represents a hydrogen atom, or a saturated hydrocarbon group having1 to 18 carbon atoms; R′₂ represents a saturated hydrocarbon grouphaving 1 to 18 carbon atoms. Most preferably, the inhibitor istert.-butyl hydroquinone (TBHQ) or tert.-butyl hydroxyanisole (BHA).

Preferably, the inhibitor is contained in the dental adhesivecomposition in an amount of from 0.01 to 0.5 mol %, more preferably inan amount of from 0.05 to 0.3 mol %.

The dental adhesive composition according to the present inventioncontains a polymerizable monomers optionally containing an acidic group.

The polymerizable monomers in the dental adhesive composition accordingto the invention are capable of free-radical polymerization and arepreferably (meth)acrylate monomers or oligomers. The dental adhesiveaccording to the invention may contain a polymerizable monomer oroligomer as a mixture of different compounds or as isomers of the samecompound. The polymerizable monomer or oligomer may include a derivativeof at least one unsaturated carboxylic acid selected from the groupconsisting of acrylic acid, methacrylic acid, cyanoacrylic acid anditaconic acid, and mixtures thereof, a derivative of styrene, or apolymerizable moiety containing a carbon-carbon double bond conjugatedwith a carbonyl group.

The (meth)acrylate monomer or oligomer is selected from materials havingat least one, and preferably two to four polymerizable double bonds permolecule so that the cured dental adhesive be crosslinked and thusbetter suited for use in the oral cavity. Monomers with a singlepolymerizable double-bond may be used in order to adjust the viscosityof the composition. (Meth)acrylate monomer materials useful herein arewell known in the art. The preferred materials generally includemonomers having a central portion containing an organic moiety and atleast two (meth)acrylic end groups. Desirable characteristics for suchmonomers and/or oligomers include good film forming properties, lowviscosity, low polymerization shrinkage, low water sorption and theability to cure rapidly and completely in the mouth. It is alsodesirable that the monomers be low in volatility and non-irritating tothe tooth pulp. A mixture of two or more appropriate methacrylatemonomers is within the scope of this invention. In fact, depending onthe choice of monomers, mixture are often highly desirable to optimizethe characteristics of the resulting dental composition.

The polymerizable monomer or oligomer may also be selected from thegroup consisting of hydroxyethyl acrylate, hydroxypropyl acrylate,diurethane dimethacrylate resin, hydroxyethyl methacrylate,hydroxypropyl methacrylate, trimethylolpropane triacrylate,1,6-hexanediacrylate, glycerin diacrylate, triethyleneglycol diacrylate,tetraethyleneglycol diacrylate, and2-acrylamido-2-methyl-1-propansulfonic acid, a reaction product ofbutane tetracarboxylic acid dianhydride and hydroxyethylmethacrylate,triethyleneglycol dimethacrylate, urethane dimethacrylate, and areaction product of butane tetracarboxylic acid dianhydride and glyceroldimethacrylate, or acrylamides or derivatives thereof such as2-acrylamido-2-methylpropane sulphonic acid,N,N-methylene-bis-acrylamide, N,N-ethylene-bis-acrylamide, and1,3-bis(acrylamido)-N,N-diethylpropane.

The polymerizable monomer or oligomer may be a phosphate based acidadhesion promoter selected from the group consisting of phosphate esteror phosphonate derivatives of radical polymerizable alcohol or polyolderivatives. The phosphate ester derivatives may be prepared using themethod given in U.S. Pat. No. 4,514,342. As examples of suitablecarboxylic acid based adhesion promoters may be mentioned the reactionproduct between butanetetracarboxylic acid dianhydride and hydroxylethylacrylate as in U.S. Pat. No. 5,218,070. Various radical polymerizableacidic monomers useful as adhesion promoters may also be obtained bymany other means, for instance as given in U.S. Pat. No. 4,806,381 andU.S. Pat. No. 6,350,839.

The polymerizable monomer or oligomer may further be a carboxylic acidbased adhesion promoter selected from the group consisting of reactionproducts between acid anhydrides and radical polymerizable derivativesof alcohols. The acid anhydride may be selected from the groupconsisting of butanetetracarboxylic acid dianhydride,tetrahydrofurantetracarboxylic acid dianhydride, benzenetetracarboxylicacid dianhydride and benzentricarboxylic acid an hydride. The radicalpolymerizable derivatives of alcohols may be selected from the groupconsisting of hydroxyethyl acrylate, hydroxypropyl acrylate, glyceroldiacrylate, pentaerythritol triacrylate, dipentaerythritolpentaacrylate, hexanediol acrylate, polyethylenoxide acrylate, andtriallypentaerythritol.

These may be mono- or polyfunctional acrylates and methacrylates, of thekind described, for example, in EP-A-0 480 472. Moreover, functionalizedmonomers with terminal acrylate or methacrylate groups may likewise beused, of the kind described, e.g., in DE-A-2 312 559 and in EP-A-0 219058.

The dental adhesive according to the present invention may containpolymerizable monomers in an amount of from 5 to 90 wt-%, preferably inan amount of from 20 to 70 wt. %.

It is preferred that the monomer or oligomer or monomer or oligomerblend has a viscosity of at most 100 Pas at 23° C., more preferably atmost 5 Pas.

The dental adhesive composition according to the invention mayoptionally comprise an organic solvent. In a preferred embodiment, thedental adhesive contains one or more further solvents selected fromconventional inert solvents such as short-chain alcohols, short-chainketones, aliphatic or unsaturated ethers, and cyclic ethersconventionally used in the dental field. Preferred solvents are selectedfrom acetone, ethanol and t-butanol. The dental adhesive composition maycontain the solvent in an amount of from 10 to 95 wt. %.

The dental adhesive composition according to the present invention mayfurther comprise an initiator. A chemical initiator may be used in caseof a multicomponent composition. The chemical initiator is able to formradicals by mixing at least two chemically different substances, whichhave to be stored separately, without a further input of energy, whichradicals are then able to initiate a polymerization reaction. Examplesof such chemical initiator systems are peroxy amine or peroxy protondonor/metal compound mixtures, of the kind described by J. M. Antonucciet al. in J. Dental Research (1979), 58 (9), page 1887-1889 or in U.S.Pat. No. 5,166,117 and in EP-A-0 115 410, 0 115 948, 0 120 559 and 0 277413. More preferred are initiators capable of forming radicals withoutmixing of different components, but based on input of energy such asthermal or light energy. In this case, the dental adhesive compositionmay be a one-component dental adhesive composition, wherein theinitiator is a photoinitiator and/or a thermal initiator. Thecomposition may comprise an alpha-diketone such as camphor quinone.

The dental adhesive may further contains an inorganic filler and/or anorganic filler; preferably the filler is a nanofiller. A filler may becontained in an amount of from 0.5 to 20 wt. %, more preferably 2 to 10wt. %.

A one-pack composition means that the composition of the presentinvention is contained in only one container which may be stored andallows application of the composition without any mixing and without anyspecial equipment before the application.

A total etch composition is a one-pack composition having priming andbonding activity on a dental surface such as dentin or enamel.

The invention will now be further illustrated with reference to thefollowing examples

EXAMPLES

Test Formulation containing different inhibitors.

A series of test formulations containing different thermalpolymerization inhibitors was prepared in order to illustrate thesurprising thermal stability of a dental adhesive composition accordingto the present invention. The standard composition was used as follows:

Component Content (wt.-%) BAP 63.2 BAA-TCD 21.1 DHPOBA_analog 54.22-Acrylamido-2-methyl- 43.8 propanesulfonic acid (AMPS) Camphor Quinone1.3 TPO 3.2 DMABE 1.5 Total 100.00 Active Matrix 55 Acrylic acid 9 Water36 Total 100

The following comparative inhibitors were tested:

(i) hydroquinone (HQ),(ii) hydroquinone monomethylether (HQME),(iii) bisphenol A,(iv) propyl gallate (PG)

The following inhibitors according to the present invention were tested:

(vii) tert-Butylhydroquinone (TBHQ), and(viii) tert.-Butylhydroxyanisol (BHA).

Test formulations containing different inhibitors or inhibitorconcentrations were stored in Prime&Bond NT bottles (Dentsply DeTrey) at60° C. until thermal polymerization. The bottles were daily examined byshaking the bottle, whereby the acoustical test turned out to be rathersensitive, and by taking a sample with a pipette. When polymerizationseemed to have occurred or after a certain minimum storage time (20days) the bottles were sliced open and the solution were examinedvisually.

According to the results of the above described Arrhenius investigationat least a thermal stability of about 11 days at 60° C. is necessary sothat the dental adhesive composition may be stored at room temperature.

Results

The Test Formulation containing different inhibitors in differentamounts, was investigated regarding its thermal stability by storingthese formulations at 60° C. The samples were daily examined. In case ofpolymerization a gel or a solid, polymerized body was observed.

The dark shaded columns represent formulations with inhibitors,respectively inhibitor concentrations, which were polymerized after thedepicted time at 60° C. The light shaded columns represent formulations,which were not polymerized until the depicted time. Usually after 20days the investigation was terminated.

In the comparison, hydroquinone (HQ) was used in an amount of 0.15 mol %showing some stabilization effect. However, hydroquinone is anallergenic compound and therefore undesirable for use in a genericdental composition. Hydroquinone monomethylether (HQME) as well as BHTfailed to provide a sufficient thermal stability.

(i) Hydrochinone (HQ)—Reference Inhibitor

(light shaded columns—formulation is not polymerized up to the recordedtime; dark shaded column: formulation is polymerized after the recordedtime)

(ii) Hydrochinone Monomethylether (HQME)—Reference Inhibitor

After slicing open the samples containing 0.49 and 0.697 mol % HQME,small pieces of gel were found at the bottom, which were not detectedbefore by shaking or by the examination with the pipette.

After slicing open the sample with 0.193 mol % TBC some pieces of gelwere found at the bottom, which were not detected before by shaking orby the examination with the pipette.

(iii) Bisphenol A—Reference Inhibitor

(the dark shaded columns indicate that the formulation is polymerizedafter the recorded time)

(iv) Propyl Gallate (PG)—Reference Inhibitor

The dark shaded columns indicate that the formulation is polymerizedafter the recorded time

(vii) tert.-Butylhydrochinone (TBHQ)—Inhibitor of the Invention

After 14 days and after 20 days at 60° C. the bottles were sliced open,the contents was investigated and filled in a new bottle, which wasstored again at 60° C. No hints of a polymerization were found.

After 20 days at 60° C. the bottles were sliced open again and thecontents was investigated. Only in case of the lowest TBHQ percentage of0.013 mol % polymerization was found. This was not detected before bythe daily examination.

(viii) tert.-Butylhydroxyanisole (BHA)—Inhibitor of the Invention

After 14 days at 60° C. all bottles were sliced open, the contents wasinvestigated and filled in a new bottle, which was stored again at 60°C. The sample with 0.047 mol % showed after 14 days at 60° C. somepieces of gel, which were not detected before by shaking or by theexamination with the pipette. After 20 days at 60° C. the bottles wereagain sliced open. No indication of polymerization for the samplescontaining 0.096 mol % and 0.147 mol % were found. The formulation with0.047 mol % again contains some small pieces of gel.

Example 1

The following non-aqueous compositions were prepared with differentamounts of TBHQ as an inhibitor. The compositions were stored at 65° C.The time required for polymerization and therefore deterioration of thecomposition was determined.

Composition/wt.-% Polymerizable Resins 67.33 67.29 67.25 67.21 CQ/Amine2.67 2.67 2.67 2.67 TBHQ 0.04 0.08 0.12 0.16 Nanofiller 5.47 5.47 5.475.47 t-Butanol 24.5 24.5 24.5 24.5 Sum 100 100 100 100 Days life topolymerization Temp./° C. t/d t/d t/d t/d 65 4 10 77 >77

It was found that 0.028-0.2 mol % TBHQ provides a sufficient thermalstability over 20 days at 65° C.

Comparison Example 1

The following non-aqueous compositions were prepared with differentamounts of BHT as an inhibitor. The compositions were stored at 65° C.The time required for polymerization and therefore deterioration of thecomposition was determined.

Composition/wt.-% Polymerizable Resins 35.35 66.87 68.11 53.15 CQ/Amine1.41 2.67 1.43 2.12 BHT 0.26 0.49 0.49 0.39 Nanofiller 2.89 5.47 5.474.35 t-Butanol 60.09 24.50 24.50 40 Sum 100 100 100 100 Days life topolymerization Temp./° C. t/d t/d t/d t/d 65 1 1 6 4

It was found that 0.028-0.2 mol % BHT does not provide a sufficientthermal stability over 20 days at 65° C.

Comparison Example 2

The following non-aqueous compositions were prepared with differentamounts of PG as an inhibitor. The compositions were stored at 65° C.The time required for polymerization and therefore deterioration of thecomposition was determined.

Composition/wt.-% Polymerizable Resins 67.31 67.26 67.21 67.16 CQ/Amine2.67 2.67 2.67 2.67 PG 0.05 0.10 0.15 0.2 Nanofiller 5.47 5.47 5.47 5.47t-Butanol 24.5 24.5 24.5 24.5 Sum 100 100 100 100 Days life topolymerization Temp./° C. t/d t/d t/d t/d 65 0-3 0-3 0-3 0-3

It was found that 0.028-0.2 mol % PG does not provide a sufficientthermal stability over 20 days at 65° C.

Example 2

0.6945 g N,N′-Bisacrylamido-N,N′-diethyl-1,3-propane, 0.2315 g3,(4),8,(9)-bis(acrylamido methyl)tricyclo-5.2.1.0^(2,6) decane, 0.0595g Ethyl 2-[12-dihydrogen phosphoryl-12,2-dioxamidecyl]acrylate, 0.0481 g2-Acrylamido-2-methyl-propane-sulfonic acid, 0.0141 g camphor quinone,0.0355 g bis(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide, 0.0164 gdimethylamino benzoic acid ethyl ester and 0.003 g2-tert-Butylhydroquinone were dissolved in a solvent mixture composed of0.1800 g acrylic acid and 0.7200 g water.

The adhesive does not polymerise after storage for 20 days at 60° C.

Example 3

0.6940 g N,N′-Bisacrylamido-N,N′-diethyl-1,3-propane, 0.2313 g3,(4),8,(9)-bis(acrylamido methyl)tricyclo-5.2.1.0^(2,6) decane, 0.0595g Ethyl 2-[12-dihydrogen phosphoryl-12,2-dioxamidecyl]acrylate, 0.0481 g2-Acrylamido-2-methyl-propane-sulfonic acid, 0.0141 g camphor quinone,0.0355 g bis(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide, 0.0164 gdimethylamino benzoic acid ethyl ester and 0.0011 g2-tert.-butyl-4-methoxyphenol were dissolved in a solvent mixturecomposed of 0.1800 g acrylic acid and 0.7200 g water. The adhesive doesnot polymerise after storage for 20 days at 60° C.

Comparative Example 3

0.6931 g N,N′-Bisacrylamido-N,N′-diethyl-1,3-propane, 0.2310 g3,(4),8,(9)-bis(acrylamido methyl)tricyclo-5.2.1.0^(2,6) decane, 0.0594g ethyl 2-[12-dihydrogen phosphoryl-12,2-dioxamidecyl]acrylate, 0.0480 g2-Acrylamido-2-methyl-propane-sulfonic acid, 0.0141 g camphor quinone,0.0354 g bis(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide, 0.0164 gdimethylamino benzoic acid ethyl ester and 0.0026 g hydroquinonemonomethyl ether were dissolved in a solvent mixture composed of 0.1800g acrylic acid and 0.7200 g water.

The adhesive polymerises after storage for 1 day at 60° C.

Comparative Example 4

0.6882 g N,N′-Bisacrylamido-N,N′-diethyl-1,3-propane, 0.2294 g3,(4),8,(9)-bis(acrylamido methyl)tricyclo-5.2.1.0^(2,6) decane, 0.0590g Ethyl 2-[12-dihydrogen phosphoryl-12,2-dioxamidecyl]acrylate, 0.0477 g2-Acrylamido-2-methyl-propane-sulfonic acid, 0.0140 g camphor quinone,0.0352 g bis(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide, 0.0162 gdimethylamino benzoic acid ethyl ester and 0.0103 g2,6-di-tert.-butyl-4-cresol were dissolved in a solvent mixture composedof 0.1800 g acrylic acid and 0.7200 g water. The adhesive polymerisesafter storage for 2 days at 60° C.

1. Non-aqueous dental adhesive comprising a mixture containing (i) oneor more polymerizable monomers optionally containing an acidic group,(ii) a polymerization initiator, and (iii) a thermal polymerizationinhibitor of the following formula (I):

wherein R′₁ represents a hydrogen atom, or a saturated hydrocarbon grouphaving 1 to 18 carbon atoms. R′₂, which may be the same or different ifmore than one R′₂ is present, independently represent a saturatedhydrocarbon group having 1 to 18 carbon atoms, and c represents aninteger of from 1 to 4, (iv) optionally an organic solvent.
 2. Thedental adhesive according to claim 1, which is stable at storage for atleast 10 days at 60° C.
 3. The dental adhesive according to claim 1 or2, wherein the saturated hydrocarbon group is a straight chain orbranched C₁₋₁₈ alkyl group or a C₃₋₈ cycloalkyl group optionallysubstituted by one or more C₁₋₅ alkyl groups or a C₄₋₈ cycloalkylalkylgroup optionally substituted by one or more C₁₋₅ alkyl groups.
 4. Thedental adhesive according to any one of the preceding claims, whereinR′₂ is a tert.-butyl group.
 5. The dental adhesive according to any oneof the preceding claims, wherein the inhibitor is TBHQ or BHA.
 6. Thedental adhesive according to any one of the preceding claims, whereinthe inhibitor is contained in an amount of from 0.01 to 0.5 mol %. 7.The dental adhesive according to any one of the preceding claims,wherein the aqueous mixture further contains an organic water solublesolvent selected from the group of alcohols and ketones such as ethanol,propanol, butanol, acetone, methyl ethyl ketone.
 8. The dental adhesiveaccording to any one of the preceding claims, wherein saidpolymerization initiator is a photo initiator such as camphor quinone.9. The dental adhesive according to any one of the preceding claims,which further contains an inorganic filler and/or an organic filler;preferably the filler is a nanofiller.
 10. Use of a compound of thefollowing formula (I):

wherein R′₁ represents a hydrogen atom, or a saturated hydrocarbon grouphaving 1 to 18 carbon atoms. R′₂, which may be the same or different ifmore than one R′₂ is present, independently represent a saturatedhydrocarbon group having 1 to 18 carbon atoms, and c represents aninteger of from 1 to 4, as a thermal polymerization inhibitor in adental composition.